Electrical power distribution systems manage the allocation of power from energy sources to electrical loads that consume distributed electrical power. In aircraft, gas turbine engines for propulsion of the aircraft typically provide mechanical energy that ultimately powers a number of different accessories such as generators, starter/generators, permanent magnet alternators (PMA), fuel pumps, and hydraulic pumps, e.g., equipment for functions needed on an aircraft other than propulsion. For example, contemporary aircraft need electrical power for electrical loads related to avionics, motors, and other electric equipment.
Over time, aircraft electrical power source voltages have increased. Aircraft with 14- and 28-volt direct current (VDC) electrical power systems have given way to aircraft with electrical power systems operating at 115 volts alternative current (VAC) and 230 VAC. Presently, aircraft can include one or more electrical power sources that operate at voltages including plus/minus 270 VDC. For example, a current wide-body twin-engine commercial jet liner uses an electrical system that is a hybrid voltage system that includes sub-systems operating at voltages of 230 VAC, 115 VAC, 28 VDC along with a bipolar, high voltage, direct current subsystem that includes plus and minus 270 VDC sources.
The voltages in the high-voltage DC electrical systems reach levels comparable to domestic AC systems. In domestic AC systems, a circuit breaker can trip to an off position, typically by way of an electromechanical switch that can actuate in approximately 50 milliseconds (ms), to de-energize the feed line when the ground current exceeds a level of 25 to 30 milliamperes (mA). But, in a high-voltage DC electrical system, similar provisions are complicated because of limited access to the ground return loop required to obtain an accurate enough measurement of current. That is, for unipolar DC voltage electrical systems, discrepancy in output current and return current is difficult to measure because the current return path from the load flows through the aircraft chassis.